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Pulling full power just because it's
there is not always a good idea.
Limitations may be there for other
reasons—for example, the
transmission might not be able to
take that much, which is why you
can’t go faster than 80 kts in a
Jetranger when pulling more than
85% torque (actually, in this case, the
transmission ends up in a strange
attitude). Excessive use of power will
therefore ruin your gearbox well
before the engine (and will show up
as metal particles in the oil).
Maximum Continuous Power is the
setting that may be used indefinitely,
but any between that and maximum
power (usually shown as a yellow arc
on the instrument) will only be
available for a set time limit.
While I'm not suggesting for a
moment that you should, piston
engines will accept their limits being
slightly exceeded from time to time
with no great harm being done.
Having said that, the speed at which
the average Lycoming engine
disintegrates is about 3450 RPM,
which doesn't leave you an awful lot
of room when it runs normally (in a
Bell 47, anyway) at 3300! Turbines,
56 Canadian Private Pilot Studies
however, are less forgiving than
pistons and give fewer warnings of
trouble because of the closer
tolerances to which they are made.
This is why regular power checks
(once a week) are carried out on
them to keep an eye on their health.
The other difference is that damage
to a piston engine caused by
mishandling tends to affect you,
straight away, whereas that in a
turbine tends to affect others down
the line. In a turbine-engined
helicopter, power used is indicated
by the torquemeter.
Apart from sympathetic handling,
the greatest factor in preserving
engine life is temperature and its rate
of change. Over and under leaning
are detrimental to engine life, and
sudden cooling is as bad as
overheating—chopping the throttle
at height causes the cylinder head to
shrink and crack with the obvious
results—the thermal shock and extra
lead is worth about $100 in terms of
lost engine life. In other words, don’t
let the plane drive the engine, but
rather cut power to the point where
it’s doing a little work. This is
because the reduced power lowers
the pressure that keeps piston rings
against the wall of the cylinder, so oil
leaks past and glazes on the hot
surfaces, degrading any sealing
obtained by compression. The only
way to get rid of the glaze is by
honing, which means a top-end
overhaul. For the same reasons, a
new (or rebuilt) engine should be
run in hard, not less than 65%
power, but preferably 70-75%,
according to Textron Lycoming, so
the rings are forced to seat in
properly. This means not flying
above 8000 feet density altitude for
non-turbocharged engines. Richer
mixtures are important as well. Also,
open the engine compartment after
shutting down on a hot day, as many
external components will have
suddenly lost their cooling. With
some turbine engines (like on the
AStar), you have to keep a track of
the number of times you fluctuate
between a range of power settings
because of the heat stress.
When levelling in the cruise, the
combination of increased speed and
throttling back cools the engine
rapidly, so close the cowl flaps
beforehand. Don't use the cowl flaps
as airbrakes, either, but to warm the
engine after starting and to cool it
after landing (allow temperatures to
stabilise before shutting down,
especially with turbochargers).
One point with low power settings
when it's very cold is that the engine
may not warm up properly and water
forming from the combustion
process may not evaporate, so oil
won't lubricate properly.
Although many flight manuals state
that as soon an engine is running
without stuttering it's safe to use it to
its fullest extent, try warming up for
a few minutes before applying any
load, at least until you get a positive
indication on the oil temperature
(and pressure) gauges. This ensures a
film of oil over all parts.
Even better, warm it before you start
it, because the insides contract at
different rates – in really cold
weather the engine block may have
the grip of death on the pistons and
strain them when the starter is
turned. Equally important is not
letting it idle when cold, as you need
it to be fast enough to create a splash
of oil inside (1,000 RPM is fine).
　
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